Granular cell myoblastoma. An electron microscopic and cytochemical study illustrating the genesis of granules and aging of myoblastoma cells.

Seven typical granular cell myoblastomas, 4 from the skin (2 multicentric) and 1 each from the tongue, vulva and breast, were studied with the electron microscope and with cytochemical procedures for the visualization of lysosomes, endoplasmic reticulum, mitochondria, Golgi apparatus and membranes. With these parameters, all of the lesions were found to be virtually identical. To the authors' knowledge, the apparent formation of the specific small granule from the Golgi apparatus and the large granule (cytolysome) by segregation of portions of cell cytoplasm as well as the apparent aging process in myoblastoma cells is described for the first time. The small granules resemble lysosomes, but do not stain with the lysosomal markers employed. The large granules (cytolysomes) contain acid phosphatase but only a few contain thiolacetic acid esterase activity, suggesting that there are at least two varieties of cytolysomes in myoblastoma. It is concluded that myoblastoma is a tumor-like lesion of Schwann cell origin, which is either a reactive cellular response or, more likely, a true neoplasm.